1. Field of the Invention
This invention relates to methods and apparatus for controlling combustion to minimize nitrogen oxide emission and, more particularly, to use of flame spectroscopy to control gas turbine combustion in a manner that reduces nitrogen oxide emissions without increased risk of flame-out.
2. Description of the Prior Art
Gas turbines are extensively used as power plants for a wide diversity of applications ranging from, for example, land based engines for gas fired electrical generators or pipeline compressors, to shipboard or airborne engines for, respectively, marine or aeronautical propulsion.
Gas turbines burn hydrocarbon fuel which may include natural gas, e.g. methane, for a land based application or kerosene, for use as an aviation (jet) fuel. As with all forms of combustion, these turbines emit an exhaust stream that contains various combustion products. While some of these combustion products, such as water vapor, are essentially harmless to the environment, others may not be and, for that reason, are classified as pollutants. Accordingly, a major effort involving, inter alia, turbine manufacturers is presently underway to produce gas turbines having significantly reduced pollutant emissions and/or to retrofit existing gas turbines so as to significantly reduce their present levels of pollutant emission. Of these pollutants, the turbine industry is especially concerned with reducing emission of various forms of nitrogen oxide, collectively referred to as "NO.sub.x ".
It is widely known that, for a gas turbine, NO.sub.x emissions increase significantly as the combustion temperature rises. It is also known that operating a turbine in a so-called "lean-burn" condition, which involves use of a lean mixture of fuel and combustion air (i.e., a relatively low fuel-to-air ratio), reduces the combustion temperature to a level that significantly reduces NO.sub.x emissions. However, if the mixture is too lean, the turbine exhibits operational instabilities which may increase to a point at which the internal combustion flame is extinguished, i.e. a "flame-out" occurs and the turbine ceases generating power. Generally, in a land based application, the turbine may be restarted after a flame-out with few, if any, adverse safety consequences even if a relatively protracted period of time is required to restart the turbine. To a large extent, this is also true for various marine applications. Accordingly, turbines designed for land (and many marine) applications can be operated under appropriate lean-burn conditions and can be restarted from time-to-time if necessary due to flame-out. However, safety concerns inherent in most aeronautical turbine applications (i.e. use of jet engines to power aircraft) require preclusion of a flame-out under certain operating conditions, such as on take-off, and, under other operating conditions, preclude attempted turbine restart over a protracted length of time.
To assure that a flame-out will not occur in an aircraft (jet) engine, the engine is frequently adjusted to operate with a "rich" fuel/air mixture, i.e., with a relatively high fuel-to-air ratio. While this results in stable engine operation, it also produces high NO.sub.x emission levels. While heretofore such high NO.sub.x emission levels have been tolerated as a cost of safe operation, environmental concerns have heightened to the point at which these emission levels need to be significantly reduced but with no ensuing diminution in operational safety.
Traditionally, most turbines rely on using a fuel to air mixture that is preset during turbine manufacture and testing to conform with an expected operating condition for the turbine, e.g. a mixture that will establish a "rich" condition for a jet engine. Apart from a throttle valve for regulating fuel flow and hence engine speed and power output, turbines generally employ no valving or other adjustments that can be used to dynamically change the turbine operating conditions, let alone change the fuel/air mixture to reduce NO.sub.x emissions.
Currently, there is no known closed-loop feedback technique for controlling a turbine, including that of a jet engine, to operate in a lean-burn condition. This is due to both the paucity of usable turbine adjustments, as noted above, as well as various difficulties associated with accurately detecting the level of NO.sub.x emissions produced by the turbine and abating these emissions in real-time and, in the case of a jet engine, without jeopardizing safe turbine operation.
At first blush, one might consider coupling a vacuum mass spectrometer, or similar device, to a gas turbine to sample the turbine exhaust and perform a spectral analysis of the sample to determine its contents by substance and concentration. Unfortunately, spectral analyzers are slow, typically requiring upwards of 10-30 seconds to generate usable results. Such time lag renders impractical use of a spectrometer or similar device to provide a real-time measurement of NO.sub.x emissions and to accurately and dynamically control a gas turbine, and especially a jet engine, to operate in a "lean-burn" condition.
Current spectral measurement based NO.sub.x abatement techniques that are thought to be suitable for use with combustion furnaces or even with land or marine based turbines present drawbacks that are so severe as to frustrate their use in jet engines. One such boiler-based technique involves directly measuring broad-band infrared radiation emitted by a combustion flame and comparing desired concentrations of various combustion products, specifically oxygen (O.sub.2), carbon dioxide (CO.sub.2), carbon monoxide (CO) and NO.sub.x, to those concentrations which actually occur in both a flue and stack. A gas mixture valve on the burner operates under programmed control to maximize, and then maintain, the infrared radiation emitted by the flame in view of measured differences between the actual and desired concentrations of these combustion products.
Rather than measuring broad-band infrared radiation, another boiler-based technique involves controlling the fuel/air ratio of multi-burner boilers based on measurements of two single spectral lines in each burner flame: an infrared carbon dioxide (CO.sub.2) line at 4.4.mu.m (micrometers) and an ultraviolet hydroxyl (OH) line at 300 nm (nanometers), respectively. An intensity ratio based on the measured values of these two lines for each burner is determined and then used to separately control the fuel/air mixture of that burner in order to achieve near stoichiometric combustion, which advantageously occurs at a fuel/air ratio that reduces the amount of NO.sub.x that heretofore has generally been emitted by multi-burner boilers. See, e.g., F. Fraim, "Research into a Spectral Flame Analyzer Phase 1--Final Report for the Period Apr. 21, 1983-Jun. 30, 1985", Work Performed under United States Department of Energy Contract DE-AC07-831D12463, Jun. 1, 1985.
Specifically, while suitable detectors, such as photodiodes, exist that can readily sense infrared radiation, the high temperature of, illustratively, an operating jet engine itself causes various engine components to emit intense amounts of radiation over the entire infrared spectrum. In addition, the jet fuel flame generally emits a large amount of unburned carbon particles which themselves function as infrared black body radiators in contrast to a "clean" natural gas flame which does not emit such particles. The resulting background level of infrared radiation is sufficiently high to completely, or almost completely, mask the radiation associated with the CO.sub.2 spectral line as well as radiation associated with other desired spectral components in the infrared spectrum. Consequently, any technique predicated on measuring radiation in the infrared spectrum produced by the flame would likely be impaired by the radiant energy produced by the jet engine itself and thus provide highly erroneous measurements.
Faced with the apparent inability to control NO.sub.x emission based upon spectral measurements, the turbine industry has turned to water injection for this purpose. By injecting water into the fuel stream, the turbine combustion temperature is reduced, thereby reducing NO.sub.x emissions. This technique, however, carries several severe practical limitations which usually render it unsuitable for use in many marine turbine applications and certainly in jet engines. Specifically, if ordinary drinking water is used, then as that water evaporates within the turbine, minerals, such as calcium, precipitate out of the water and form deposits over various internal components. If these deposits are allowed to accumulate, the close mechanical tolerances between adjacent internal components will cause these components to bind and thereby substantially reduce the turbine service life. To avoid this type of damage, one of two approaches is typically taken: either the turbine is routinely removed from service to undergo removal of accumulated deposits, or purified water injection is used. The former approach is likely to be costly both in terms of turbine downtime and maintenance expense and, for that reason, is generally avoided. The latter approach, while relatively simplistic, often requires use of either appropriate water purification equipment with an attendant energy source and equipment maintenance requirements, or a suitably large tank or other source to supply a continual source of demineralized water. A stationary land based turbine is usually sited near an ample supply of such water or at a location which can accommodate the extra space required for either this equipment and/or a tank. Available space for such purification equipment or a tank is clearly at a premium aboard a marine vessel and simply nonexistent aboard an aircraft. Furthermore, various by-products of water purification, such as precipitated minerals and the like (including salt if sea water is being purified) can present a disposal problem in and of themselves. Accordingly, water injection is rarely used for marine applications and not at all for jet engines.
Thus, a need exists for a technique that can substantially reduce turbine NO.sub.x emissions by operating the turbine in a "lean-burn" condition. Such technique should not rely on either water injection or detection of infrared emissions, and should provide closed-loop feedback control to assure safe, stable turbine operation by preventing unintended "flame-out". In addition, the technique should be readily amenable to inclusion in existing turbines, on a retrofit basis, as well as to inclusion in new turbines during their manufacture.